Electrical switching device and method of embedding catalytic material in a diamond substrate

ABSTRACT

An electrical switching device ( 30 ) is disclosed. The device comprises a diamond substrate ( 24 ), a cathode ( 34 ) in contact with the substrate and having electrically conductive emitters ( 32 ) extending into the substrate, and an upper electrode ( 36 ) in contact with the substrate and spaced from the cathode.

The present invention relates to electrical switching and amplificationdevices, and relates particularly, but not exclusively, to electricalswitching devices for use in high power applications. The invention alsorelates to a method of embedding catalytic material in a diamondsubstrate.

As is well known by persons skilled in the art, silicon has limitationswhen used as a base electronic material for switching and amplificationapplications in high power and extreme environments. For example,silicon is known to have a reverse breakdown voltage of about 8 kV,based on acceptable levels of on-state losses and usable switchingspeed. This means that to achieve significant voltage and/or currentlevels in several end use applications requires employing a plurality ofdiscrete devices in series or parallel combinations, which in turnrequires other electronic components to ensure that the duty betweendevices remains balanced and hence can be complex to undertake.

Diamond offers potential solutions to this problem because of itsthermal, dielectric and carrier mobility properties which are generallysignificantly better than any other electronic material. Devices madefrom diamond hence offer the potential to significantly reducecomplexity by reducing the number of discrete devices needed to fulfilthe application.

In most semiconductor devices, electronic function is achieved bymodifying the electronic properties of the base material through theselectively introduction of foreign elements into the crystallinestructure otherwise known as dopants. In the case of diamond the choiceof dopants is limited by the comparatively small size of the crystallattice. The result is that the two dopants that create the leastdisturbance to the diamond crystal structure are boron (p-type) andnitrogen (n-type). While boron doped diamond is a reasonably effectivep-type semiconductor, no effective n-type dopant has yet been found. Theproblem is that both dopant species are deep donors with activationenergies of 0.7 eV and 4.5 eV respectively leading to the requirementfor heating to assist the release of charge carriers in order to realiseeffective device operation.

However the process of heating also leads to a reduction in carriermobility values and electric field breakdown strength thus compromisingtwo of the key properties that make diamond ideal for the fabrication ofhigh power switches. As such devices fabricated in this way do notexploit the inherent physical properties of diamond as an electronicmaterial.

Existing devices always trade-off the voltage that they can block in theoff-state with the current that they can pass in the on-state. Furtherto achieve the highest voltage and power ratings in non-diamondmaterials usually requires a bipolar structure for the device. As thejunction size is physically increased to increase the switching voltageof the device the amount of charge that has to neutralised to turn thedevice off also increases, which in turn compromises the switchingfrequency of the device.

Preferred embodiments of the present invention seek to overcome theabove disadvantages of the prior art by using a unipolar structure thatenables the full material potential of diamond to be exploited for powerdevices. The present invention exploits a number of known phenomenarelating to diamond. Namely that: ballistic electrons can travel througha few hundred micrometres of intrinsic diamond with very little loss;doped diamond can be used to control space charge build-up from amono-species plasma; and, electrons can be emitted into diamond directlyfrom a sharp conductive intrusion embedded within a diamond substrate.The invention therefore seeks to employ an engineered electron emitterstructure embedded within diamond that in turn will only conduct in thepresent of a suitable direct current bias and to use the electrontransport properties of diamond to enable conduction through thematerial.

According to an aspect of the present invention, there is provided anelectrical device comprising:

a substrate including at least one diamond layer;

at least one first electrode in contact with said substrate, wherein atleast one said first electrode includes at least one electricallyconductive protrusion extending into said substrate; and

at least one second electrode in contact with said substrate and spacedfrom the or each said first electrode.

This provides the advantage that by suitable construction of theelectrically conductive protrusions, field-enhanced electron emissioncan occur within the diamond material. This in turn enables the deviceto operate at lower temperatures than conventional electrical devicescontaining diamond, and to have favourable on-state conductivity andoff-state current blocking properties.

The substrate may comprise, adjacent at least a distal end of at leastone said protrusion, diamond material including first impurities adaptedto modify the electrical characteristics of said diamond materialadjacent said distal end.

This provides the advantage of reducing the potential barrier betweenthe electrically conductive protrusions and the diamond layer, which inturn reduces the potential required to activate the device.

The first impurities may be adapted to provide said material with n-typeelectrical characteristics.

This provides the advantage of introducing free electrons which enhancethe conductivity of the device in its on-state.

The substrate may comprise, adjacent at least one said second electrode,diamond material including second impurities adapted to modify theelectrical characteristics of said diamond material adjacent said secondelectrode.

This provides the advantage of enabling the incidence of space chargebuild-up within the substrate material to be reduced, which wouldotherwise restrict the magnitude of the on-state current of the device.

The second impurities may be adapted to provide said diamond materialwith p-type electrical characteristics.

The device may further comprise at least one third electrode arranged insaid substrate and spaced from the or each said first electrode and theor each said second electrode.

This provides the advantage of enabling the electric field in thevicinity of the protrusions, and therefore the current flow in theon-state, to be controlled by applying a suitable voltage bias to thethird electrode.

At least one said third electrode may define at least one respectiveaperture adjacent at least one said protrusion.

This provides the advantage of enabling further control of the electricfield in the vicinity of the protrusions.

At least one said third electrode may be arranged in a layer ofintrinsic diamond material.

This provides the advantage of reducing the leakage current from thethird electrode to the first or second electrodes, which in turnimproves performance of the device.

At least one said third electrode may include non-diamond carbon formedthrough the transformation of selective areas of the diamond materialusing an implantation technique.

This provides the advantage of simplifying manufacture of the device, byminimising the requirement for buried metallic layers.

At least one said third electrode may include diamond materialcontaining impurities adapted to increase the electrical conductivity ofsaid material.

This provides the advantage of avoiding having to graphitise diamond byexploiting the high mobility properties of semiconductor diamond and maybe homoepitaxially grown rather than implanted.

The device may further comprise a plurality of separated said secondelectrodes.

This provides the advantage of enabling devices to be constructed inwhich current flow between a plurality of second electrodes can becontrolled by the application of a suitably biased voltage signal to thethird electrode, as a result of which amplification can be achieved.

According to another aspect of the present invention, there is provideda method of modifying a substrate including at least one layer ofdiamond material, the method comprising:

depositing at least one catalytic material on at least predeterminedregions of diamond material of said substrate;

causing at least part of said diamond material in contact with at leastpart of said catalytic material to be converted into non-diamond carbon;and

causing at least part of said catalytic material to penetrate saidsubstrate.

This provides the advantage of enabling long holes of small diameter tobe created in the substrate. This in turn enables electrical devices tobe manufactured having electrically conductive protrusions with highaspect ratio, which in turn enables devices of high performance to bemanufactured.

The method may further comprise causing said catalytic material on saidpredetermined regions of said diamond material to form separated regionsof catalytic material.

This provides the advantage of enabling the catalytic material to bepatterned on the substrate in a highly localised manner.

The step of causing at least one said catalytic material to form aplurality of separated regions may comprise heating said material in thepresence of at least one plasma discharge containing at least onereducing component.

At least part of said diamond material in contact with at least part ofsaid catalytic material may be caused to be converted into non-diamondcarbon by means of heating.

The method may further comprise modifying at least part of a surface ofsaid substrate prior to depositing said catalytic material thereon toreduce reactivity of said catalytic material with said substrate.

This provides the advantage of enabling reaction of the catalyticmaterial with the diamond substrate to be controlled, which in turnimproves the manufacturing process.

The method may further comprise forming at least one region ofnon-diamond carbon damage to diamond material on a surface of saidsubstrate.

This provides the advantage of aiding localisation of the reaction ofthe catalytic material with the substrate, which in turn improves theaccuracy of the manufacturing process.

At least one said catalytic material may be patterned by means of alithographic process.

At least part of said catalytic material may be caused to penetrate saidsubstrate by means of heating the substrate to a sufficient temperatureto initiate catalytic decomposition of the diamond to non-diamond andapplying a suitable uniformly biased magnetic and/or electric field.

This provides the advantage of ensuring catalytic action and impartingdirectional movement to the catalytic material through the interactionof the catalytic material and the external electric and or magneticfield.

At least part of said catalytic material may be caused to penetrate thesubstrate by means of at least one plasma discharge.

This provides the advantage of imparting electrical charge to thecatalytic material to ensure that interaction with the external electricand or magnetic field will direct the catalytic material to penetrateinto the substrate by applying a suitable dc bias to the plasmadischarge.

The method may further comprise removing non-diamond carbon by means ofat least one plasma discharge by utilising gaseous species that areknown to preferentially react with non-diamond carbon that may beoptionally used as the base plasma to impart charge to the catalyticmaterial.

This has the advantage of reducing the number of manufacturing steps.

The method may further comprise modulating at least one said plasmadischarge using an alternating current source such as a radio ormicrowave frequency supply.

This provides the advantage of reducing ion impact damage to the diamondmaterial but will enhance the reactive removal of non-diamond carbon.

According to a further aspect of the present invention, there isprovided a method of manufacturing an electrical device, the methodcomprising:

forming at least one hole in a substrate by means of a method as definedabove;

forming at least one first electrode in contact with said substrate; and

forming at least one second electrode in contact with said substrate andspaced from the or each said first electrode.

Preferred embodiments of the invention will now be described, by way ofexample only and not in any limitative sense, with reference to theaccompanying drawings, in which:

FIGS. 1A to 1I show steps of a manufacturing method embodying thepresent invention;

FIG. 2 is a schematic cross-sectional representation of an electricalswitching device of a first embodiment of the present invention;

FIG. 3A is a schematic representation, corresponding to FIG. 2, of adevice of a second embodiment of the present invention;

FIG. 3B is a plan view of a control electrode of the device of FIG. 3A;

FIG. 4 is a schematic representation, corresponding to FIG. 2, of adevice of a third embodiment of the present invention;

FIG. 5 is a schematic representation, corresponding to FIG. 2, of adevice of a fourth embodiment of the present invention; and

FIG. 6 is a schematic representation, corresponding to FIG. 2, of adevice of a fifth embodiment of the present invention that incorporatesfeatures of the third and fourth embodiments.

Referring to FIG. 1A, a diamond substrate 2 is polished to a high degreeof flatness and may be treated using techniques familiar to thoseskilled in the art to form an oxygenated, hydrogenated, halogenated orhydroxyl supporting surface on the substrate 2. This process may involvea high frequency glow discharge (not shown). A catalytic metal 8 such asiron, cobalt, nickel or platinum is then deposited as a uniform layer onthe treated surface of the substrate 2 by means of an evaporation orsputtering process to a thickness of 2 to 50 nm, and a mask 3 ofsuitable material is then deposited on the catalytic metal B. Referringto FIG. 1B the mask 3 may be selectively removed using a lithographicprocess to leave in the desired pattern 31 and expose the remainingareas of catalytic metal 8.

The catalytic metal 8 not covered by the mask 3 is then removed by meansof a chemical etching process to provide separated regions 81 ofcatalytic metal as shown in FIG. 1C.

Alternatively the steps shown in FIGS. 1A through 1C may be reversedsuch the mask 3 is deposited directly on the substrate 2 andlithographically processed to achieve the desire pattern 31 prior todeposition of the catalytic metal 8 to achieve the separated regions 81.

The remaining mask 31 is then removed as shown in FIG. 1D, and coatedsubstrate 2 is annealed at 500° C. or higher in a vacuum environment orin the presence of an ac modulated plasma discharge 12 containing ahydrogen rich gas such as ammonia at a pressure of 1-10 mbar struckbetween electrodes 4 and 5 as shown in FIG. 1E, which causes theseparated regions 81 of catalytic metal to form highly localisedglobules or spheroids 82 as shown in FIG. 1F.

Referring to FIG. 1G, on completion of the annealing process, thesubstrate 2 is heated to a temperature of 600° C. or more andsimultaneously a uniform electric field with a bias voltage of 400V ormore is applied by means of a DC power supply 7 applied to electrode 41such that it is positively biased relative to an electrode 51 located onthe opposite face of the substrate 2. Under these conditions, thecatalytic particles 82 cause the diamond of the substrate 2 in contactwith the particles 82 to be converted to non-diamond carbon, such asgraphite or amorphous carbon.

By introducing an atmosphere of a hydrogen rich gas such as ammonia at apressure of 1-10 mbar a plasma discharge 13 will be struck betweenelectrode 41 and the substrate 2. The plasma discharge 13 causes thecatalytic particles 82 to acquire an electrical charge because of theirsmall size, and the bias voltage of DC supply 7 causes the catalyticparticles 82 to move in the direction of the field towards the electrode51, i.e. further into the substrate 2 leaving a trail of non-diamondcarbon. By ensuring that the voltage drop across the plasma dischargebetween electrodes 41 and 51 exceeds 400V the trail of non-diamondcarbon (not shown) will be columnar and orientated according to thefield direction. The efficacy of this action is further enhanced by theintroduction into the discharge 13 of additional gases known to thoseskilled in the art that will preferentially react with non-diamondcarbon and thereby etch the non-diamond carbon generated by the actionof the catalytic particles 82 to create small diameter holes 20. A radiofrequency power supply 22 may also be introduced to modulate thedischarge 13, which further enhances the process of etching the graphiteabove the catalytic particles 82 to remove the graphite by means of thedischarge 13. When holes 20 of the desired depth are created, thesubstrate 2 is allowed to cool, as a result of which the catalyticaction of particles 82 ceases and the modified substrate 24 shown inFIG. 1H is formed.

The modified substrate 24 is then cleaned by means of a suitable acid toremove the catalytic particles 82 from the bases of the holes 20, and isthen optionally further treated by means of a compound such as moltensodium nitrate to remove any remaining amorphous or graphitised carbon.A further layer 14 of diamond can then be grown on the non-etched faceof the substrate 24 to achieve the desired electrical and or mechanicalproperties, as shown in FIG. 1I.

Referring now to FIG. 2, a diode-type electrical switching device 30 isformed using an intrinsic or nitrogen doped diamond substrate 24 asshown in FIG. 1I. The holes 20 in the substrate 24 are filled with asuitable electrically conductive metal to form a metal micro-emitterstructure 32 in contact with a main cathode 34. Alternatively, more thanone metal may be deposited in steps in the holes 20 in order to make themetal/diamond interface ohmic in nature or to reduce the work functionbetween the metal and diamond, especially in the vicinity of the holes20. Prior to deposition of these metals the holes 20 and substrate 24may be pre-treated in an oxygen and or hydrogen high-frequency discharge(not shown) to assist in further reducing the potential barrier betweenthe metal 32 and substrate 24. A further plane electrode 36 is placed onthe opposite plane surface of the diamond substrate 24 and is insulatedfrom the ends of the emitters 32 by the layer 14 of intrinsic or lowboron doped homoepitaxial diamond or other such dopant so as tointroduce electron acceptor sites within the layer 14. When a voltage isapplied between the electrodes 34, 36 such that the potential of theupper electrode 36 is greater than that of the lower electrode 34 andsuch that the voltage is of sufficient magnitude so that the local fieldat the ends of the emitters 32 is sufficient to cause the emission ofelectrons into the diamond layers 24 and subsequently 14, electricalconduction between the electrodes 34, 36 can occur. If the bias of powersupply 38 is reversed, on the other hand, such that the potential of thelower electrode 34 is greater than that of the upper electrode 36, theemission of electrons is significantly reduced, since the curvature ofthe surface of the plane electron emitting electrode 36 is significantlyless than that at the distal ends of the emitters 32.

A device 130 of a second embodiment of the invention is shown in FIG.3A, in which parts common to the embodiment of FIG. 2 are denoted bylike reference numerals but increased by 100. The forward and reversebias characteristics of the diode device 30 shown in FIG. 2 areprimarily determined by the thickness of the insulating layer 14. Sincea large voltage drop would occur across the device of FIG. 2 as a resultof the size of the field required to sustain electron emission, thiswould limit the performance of the device as a rectifier. However, theembodiment shown in FIG. 3A alleviates this problem by means of controlof injection of electrons into the substrate 124 by means of theelectric field established between a gate electrode 140 and the emitters132. The gate electrode 140 is manufactured by means of first growing atthe stage indicated by FIG. 1I an additional intermediate layer ofintrinsic diamond 126 on the base substrate 124 to which a maskedimplantation process is applied such that a subsurface layer of thediamond is treated using a high energy ion beam to create a conductivelayer in the diamond prior to growing the additional diamond layer 114.The conductive layer is patterned as shown in FIG. 3B, with apertures127 centred around the tip of each emitter 132, whose alignment isindicated by a cross in the figure, to produce a generally uniformannular field about the tip of each emitter 132. The current flowbetween the upper 136 and lower 134 electrodes will be generallyproportional to the applied voltage between the gate electrode 140 andthe emitters 132.

A device 230 of a third embodiment of the invention is shown in FIG. 4,in which parts common to the embodiment of FIGS. 3A and 3B are denotedby like reference numerals but increased by 100. At the stage indicatedby FIG. 1I a further layer 246 of a heavily nitrogen doped or othern-type doped layer 246 of diamond is grown or implanted on top of thetreated diamond layer 226 as described in the third embodiment. A finallayer 214 of intrinsic or p-type diamond is then grown on the top of then-type layer 246. The purpose of the n-type layer 246 is to diffuseelectron emission from the point sources formed by the tips of theemitters 232, as a result of which better use is made of the availablebulk diamond. The n-type layer 246 also helps screen the emitter tips232 and gate 240 when the device is reverse biased.

A device 330 of a fourth embodiment of the invention is shown in FIG.5A, in which parts common to the embodiment of FIG. 4 are denoted bylike reference numerals but increased by 100. The upper electrode 236 ofFIG. 4 is replaced by two separated metallic electrodes 350, 352 and afurther insulating layer 354 is optionally grown or deposited on thesubstrate 324 to partially or completely cover the electrodes 350, 352and the space between the electrodes. If a voltage is applied solelybetween the electrodes 350 and 352, no current results. However, if aseparate bias is applied to gate electrode 340 and the emitters 332 suchthat electrons are emitted into the additional diamond layer 314,conduction between the electrodes 350, 352 becomes possible by virtue ofthe availability of carriers generated by electrons emitted from theemitters 332. The magnitude of electrical current flow between theelectrodes is determined by the number of electrons injected into thediamond layer 314, which is in turn generally proportional to the biasapplied between the gate electrode 340 and the lower electrode 334.

A further embodiment of the invention is shown in FIG. 6, whichincorporates features of the third and fourth embodiments to thisinvention in which parts common to the embodiment of FIGS. 4 and 5 aredenoted by like reference numerals but increased by 100. The device 430includes an n-type layer 346 and p-type layer 414 similar to thearrangement of FIG. 4. The presence of the n-type layer 346 helps tomaintain a generally uniform density of electron injection into thediamond layer 414 and deters leakage current to the emitter electrodes432 and gate electrode 440.

It will be appreciated by persons skilled in the art that the aboveembodiments has been described by way of example only, and not in anylimitative sense, and that various alterations and modifications arepossible without departure from the scope of the invention as defined bythe appended claims. For example, the substrate 24 of the invention maybe used for a purpose other than as the basis for a switching electricaldevice, for example as a fine mesh filter or optical waveguide.

1-23. (canceled)
 24. A method of modifying a substrate including atleast one layer of diamond material, the method comprising: depositingat least one catalytic material on at least predetermined regions ofdiamond material of said substrate; causing at least part of saiddiamond material in contact with at least part of said catalyticmaterial to be converted into non-diamond carbon; and causing at leastpart of said catalytic material to penetrate said substrate.
 25. Amethod according to claim 24, further comprising causing said catalyticmaterial on said predetermined regions of said diamond material to formseparated regions of catalytic material.
 26. A method according to claim25, wherein the step of causing at least one said catalytic material toform a plurality of separated regions comprises heating said material inthe presence of at least one plasma discharge containing at least onereducing component.
 27. A method according to claim 24, wherein at leastpart of said diamond material in contact with at least part of saidcatalytic material is caused to be converted into non-diamond carbon bymeans of heating.
 28. A method according to claim 24, further comprisingmodifying at least part of a surface of said substrate prior todepositing said catalytic material thereon to reduce reactivity of saidcatalytic material with said substrate.
 29. A method according to claim24, further comprising forming at least one region of non-diamond carbondamage to diamond material on a surface of said substrate.
 30. A methodaccording to claim 24, wherein at least one said catalytic material ispatterned by means of a lithographic process.
 31. A method according toclaim 24, wherein at least part of said catalytic material is caused topenetrate said substrate by means of heating the substrate in thepresence of a dc biased magnetic and/or electric field.
 32. A methodaccording to claim 31, wherein at least part of said catalytic materialmay be caused to penetrate the substrate by means of at least one plasmadischarge.
 33. A method according to claim 24, further comprisingremoving non-diamond carbon by means of at least one plasma discharge.34. A method according to claim 33, further comprising modulating atleast one said discharge.
 35. A method of manufacturing an electricaldevice, the method comprising: forming at least one hole in a substrateby means of a method according to claim 24; filling at least on holewith an electrically conductive material; forming at least one firstelectrode in contact with said substrate and filled hole; and forming atleast one second electrode in contact with said substrate and spacedfrom the or each said first electrode.
 36. An electrical devicecomprising: a substrate including at least one diamond layer; at leastone First electrode in contact with said substrate, wherein at least onesaid first electrode includes at least one electrically conductiveprotrusion extending into said substrate; and at least one secondelectrode in contact with said substrate and spaced from the or eachsaid first electrode.
 37. A device according to claim 36, wherein thesubstrate comprises, adjacent at least a distal end of at least one saidprotrusion, diamond material including first impurities adapted tomodify the electrical characteristics of said diamond material adjacentsaid distal end.
 38. A device according to claim 37, wherein the firstimpurities are adapted to provide said material with n-type electricalcharacteristics.
 39. A device according to claim 36, wherein thesubstrate comprises, adjacent at least one said second electrode,diamond material including second impurities adapted to modify theelectrical characteristics of said diamond material adjacent said secondelectrode.
 40. A device according to claim 39, wherein the secondimpurities are adapted to provide said diamond material with p-typeelectrical characteristics.
 41. A device according to claim 36, furthercomprising at least one third electrode arranged in said substrate andspaced from the or each said first electrode and the or each said secondelectrode.
 42. A device according to claim 41, wherein at least one saidthird electrode defines at least one respective aperture adjacent atleast one said protrusion.
 43. A device according to claim 41, whereinat least one said third electrode is arranged in a layer of intrinsicdiamond material.
 44. A device according to claim 41, wherein at leastone said third electrode includes non-diamond carbon.
 45. A deviceaccording to claim 41, wherein at least one said third electrodeincludes diamond material containing impurities adapted to increase theelectrical conductivity of said material.
 46. A device according toclaim 36, further comprising a plurality of separated said secondelectrodes.